Laterite nickel and cobalt ore deposits generally contain oxidic type ores, limonites, and silicate type ores, saprolites, as two layers in the same deposits, separated by a transition zone.
The higher nickel content saprolites tend to be treated by a pyrometallurgical process involving roasting and electrical smelting techniques to produce ferro-nickel. This treatment normally involves a drying step, followed by a reduction roast step to partially convert the nickel oxides to nickel, and smelting in an electrical furnace. This is a highly energy intensive process and requires a high grade saprolite source to make it economic. It also has the disadvantage that financial value of any cobalt in the ore, which is recovered into the ferro-nickel, is not realised.
The high nickel and cobalt content limonite is normally commercially treated hydrometallurgically by the High Pressure Acid Leach (HPAL) process using sulphuric acid in which iron is precipitated as hematite as ferric oxide, or by a combination of pyrometallurgical and hydrometallurgical processes, such as the Caron reduction roast-ammonium carbonate leach process.
Other acid leach processes for extracting nickel and cobalt from laterites are described in the literature. These include atmospheric pressure acid leaching, separately leaching the limonite and saprolite fractions by combinations of high pressure and atmospheric leaching, and heap leaching. In these acid leach processes sulfuric acid is usually the acid of choice, but the use of hydrochloric acid, or organic acids has also been described. As the iron in the ores treated is in the ferric state, the leached iron is precipitated as jarosite, goethite, ferrihydrite, hematite or iron hydroxide, depending on the technology used. The relevant recovery methods for nickel and cobalt described are also limited to the treatment of liquor containing ferric as the unique iron component.
A common feature in atmospheric pressure acid leaching is that a substantial portion of the high iron content of the laterite leaches along with the nickel and cobalt, and reports as ferric ions in the product leach solution (PLS), and current processes for treatment of the (PLS) focus on the recovery of target metals such as nickel, cobalt and occasionally copper from the leachate containing ferric as the dominant iron component.
Heap leaching is a conventional method of economically extracting metals from ores and has been successfully used to recover materials such as copper, gold, uranium and silver. Generally it involves piling raw ore directly from ore deposits into heaps that vary in height. The leaching solution is introduced onto the top of the heap to percolate down through the heap. The effluent liquor is drained from the base of the heap and passes to a processing plant where the metal values are recovered.
Heap leaching of laterites is taught in U.S. Pat. No. 5,571,308 (BHP Minerals International, Inc), which describes a process for heap leaching of high magnesium containing laterite ore such as saprolite.
U.S. Pat. No. 6,312,500 (BHP Minerals International, Inc) also describes a process for heap leaching of laterites to recover nickel, which is particularly effective for ores that have a significant clay component such as nickel-containing smectite and nontronite (greater than 10% by weight).
A major problem with the heap leach process is that the leachate produced contains, in addition to the nickel and cobalt values targeted, large quantities of ferric iron ions and a variety of other impurities. The purification of similar nickel solutions from commercial laterite acid leach processes involves neutralisation of the acid content, precipitation of ferric iron ions, followed by production of a nickel/cobalt intermediate, a re-dissolution step, and complex solvent extraction stages to produce saleable nickel and cobalt. The purification steps generally aim for complete removal of iron and the other impurities.
Ion Exchange (IX) processes have been disclosed for the extraction of both the nickel and cobalt from the nickel leachate, leaving the major impurities in the raffinate.
US Patent 95/16118 (BHP Minerals International Inc.) describes an ion exchange process for separating nickel from the leachate from treatment of laterite by the pressure acid leach process. Nickel is extracted by the resin at pH less than 2, and stripped with sulfuric acid for subsequent electrowinning. Cobalt remains in the raffinate along with other impurities, and after solution neutralisation, is precipitated as a sulfide.
Patent WO 00/053820 (BHP Minerals International Inc.) describes the ion exchange extraction of nickel and cobalt from acid sulfate leach solution onto the resin, and the subsequent acid stripping of the metals from the resin, and their separation by solvent extraction.
U.S. Pat. No. 6,350,420 B1 (BHP Minerals International Inc.) also teaches the use of ion exchange resin in a resin in pulp process to extract nickel and cobalt onto the resin from an acid leach slurry.
The preferred resin used in these patents is Dow M4195 which has the functional group bis-picolylamine and the adsorption constants indicating selectivity of the resin at pH 2 are in the order of Cu2+>Ni2+>Fe3+>Co2+>Fe2+>Mn2+>Mg2+>Al3+. The above patents all aim to produce relatively pure nickel solution, or nickel and cobalt strip solutions from the ion exchange resins.
An improvement to the ferro nickel process described earlier is taught in International Patent application (PCT/AU.2005/001360) (BHPBilliton SSM Development Pty Ltd) which teaches a method of producing a nickel/iron hydroxides to feed the smelting step. This involves heap leaching of the laterite, an ion exchange stage with Dow M4195 to separate nickel and some of the iron from the ferric ion containing product liquor. As ferric ion concentration in the PLS produced is ten times the concentration of the nickel ions, the effective resin capacity for nickel adsorption is decreased due to the loading of the ferric ions.
It has been surprisingly found following detailed experimental work and pilot plant operation, that contrary to what is taught in prior art, some partially oxidised laterite ores which are less weathered, or have a younger geological history, contain a substantial proportion of their iron content in ferrous form, and when acid leached in a heap leach process to recover nickel, generate a product leach solution in which most of the iron is in ferrous form. This discovery has required a changed philosophy for iron treatment in hydrometallurgical processes in the laterite industry, and has led to the process of the present invention which overcomes or at least alleviates one or more of the difficulties associated with the prior art.
The above discussion of documents, articles and the like is included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date.